Playback circuit for magnetic recordings



March 31, 1953 l H. M. FLEMING, JR PLAYBACK` CIRCUIT FOR MAGNETIC RECORDINGS Filed Nov. 30

vCLOSED PAC/Eb spors SIGNAL AS F/RS T AMPL/F IED, L/M/ TED AND SHAPED orLArL-o Our/ur FROM L] LAST $72165 OF-AMPL/HER in if? m GAT/NG /NSMNTS PER/PHERY 0F bkuM NOTE: GATES 20 0F F/G.2 ARE OPENED /NVENTOR A7' INSTANTS SHOWN IN F/GJ.

By HOWARD M FLEM/NG, JR.

ATTORNEK .integral pulse-'time delay;.

Patented Mar. 31,- 1953` This nyention [relates o playbackcircuits for magnetic recerdings.` It 1 is' particularly,v useful in f the; playback of .coded pulse recordingsY as niadefofr 'electr'onicfcomputers; The g coded inmmailon; may;be of literalA ornumerica1 sig,- .niflcanc For computing operations the binary system fgenllmcrationisfrequent1y;;used,; or else acoded de cimalfsysem wherein each of the ten fp-,digits f is; represented` i by an appropriate- 4- elenient;.;code 1 combination. The present inventio'nffisf` generally useful forxiplayback of `pulse recordings:` regardless: of the -code system that mayfsqbejadopted; Y

f'f-Whenf the playback iszto `bei` obtained byA scanning amagnetic recordingltrackfon thefperiphery of a constantly revolving drum itp is l a well vknown practice to first-record the'pulses in Wellidelined andevenly "spaced spots, the location Iofwhich iszdetermined; by means of yprecisely timed gating pulses emanating from a pulse generator. v'The frequency of. the generator is a function' of the revolution speed of't-he KAdrum. The instant of occurrence of awgating *pulse :wil-l lbe referred toihereinafterasra record time. The cadence atwhich recorldltmes are recurrent will be under` stood'"tog-.correspond withV thefrate of scanning of adjacentfmagnetizationfspots on the drumigi Certain'requirements of an electronic computer-` include l the :playback-` of information as previously; recorded on ya# magnetic drum. If

thezpulsefoutput;from-a playbackcircuitis found to` driftiaway from; the vrecord times;Y then com:l plicationsf'arise in'y the; utilization of -the playback signa1s: 'jThe phase shift ofthe vplayback signals presentlygto.; be described;` The principalnchar- `acteristic; o fzthis; means .is that it enables the playback circuit; 4to deliver pulses `i n,,exact .syn-

chronis'm with the record times andvwithout-any t A secondary@ 'object fis to fpi'ovide means; Y for the Ccocieiil data', may be f` readilyyutilizedsin subsequent computingoperationsor orfvisually in@V -Y practical limltto-th densityi'offspot A, alssei 4VPLAYBACK- CIRCUIT Fon MAGNETIC gf yimcoimINds y "Howard yM;Fleming,Jr., Orange, NLJ., assigner tof Monroe 'Calculating Machine Company, Orange, vN; J.,"a` corporationof Delaware ,i j y I i i 44Application"Novemberso;195o,serial10.198391 'l .ecimsif (01.340-174') krecordings on the -periphey ofa magnetid This limit varies as a y'function partly" of the peripheral speed and partlyfof the brevityfof the recording pulse. But itis more dependent upon anunavoidable spready ofthe linesdf frcev cent thepole'pie'ces of the scanning. head; 1;- `Magnetic `recording.; .techniques have nowibe'en developed to the point wherelittle"further` gain can be expected in minimizing they spread, or fringe effect`of the magnetiation spots. V The assumed optimum ofthe spot density4 factoiirhaving Vbeen reached empirically, I havefurid it Y possible to `take advantage `of Athis fringe effect.

The potential induced in the playback head builds up gradually as aresult ofn the fringe'e'ffect and while the magnetization. spot `approachesithe magnetic axis ofthe-pole pieces .inthescan- 1 ning head.` The voltageybeing proportional to Y .the rate Vof change ofthe l magneticflux, [levels off andv reverses itself atv the instant Whenscanningthe approximate centerA of ythespot.. The playback circuit arrangementiwhich isjof ythe essence ofi-my invention is particularly `useful in conjunction with electronic computers `and the like, because it enables gating operations to be performed at the yexact instant ofpassage of Y the magnetization spot Vvacross the gap between the pole` pieces of lthe scanning head. Thus, a succession of different Yoperations of. anfelectronic computer is capable of being vperformed While maintaining thediierent gating instants in step,` lor vin synchronism with a-basicgating pulsecycle;g- -j-4 Y, x-

ix My invention--will now be described-in-more detail, referencebeingmadeto the accompanying drawinssin which f Fig. 1 showsfour curves a, b,- c and d'whi'ch are related to one `another and 'are yall drawn to the same horizontal time scale. Theyindicate Wave shapes of signalsdealt with in a scanning operation; curve a being under diierentfcondb .tions than those ofthe otherscurves;

Fig. 2 v`is -a schematic circuit jdiagram illustrativeyof apreferred. embodiment Vof thehinvention. 1 f 1 e Referringiirstto Eigi l, curve a therein shows a typical signal- Wave as generatedin anp1ayb-ack scanning head when scanning'va` series-of ,recorded pulses on -a magnetic drum, the spacing of the magnetized spotsbeilg s0 separatedtthat there isno overlap of the fringe effect` Qurveb shows" 'ra f modified `form -ofisignal-wave produced Vin `thesamev mannerfbut` with a H closer packing of'lthelfmagnetized lspotonthedrurn Here there is ani overlapV of tithe fringeeffect suilicientto prevent the return of the potential to zero level between the trailing end of one pulse and the leading end of a succeeding pulse of opposite polarity.

In curve b of Fig. 1 it will be observed that each full cycle of the potential curve of the signal wave is smoothly joined to another cycle, provided that they represent the scanning'of successive spots of the same polarity. Assuming that the recording is a storage of binary digits, then the graphs represent the binary'numeral 11010. The magnetized spots produce a leading end half cycle of negative polarity to .indicate numeral l and of positive polarity to indicate numeral Where the record .of .a 10 follows that of a 1 there is a slight dip between positive potential peaks, this ldip being of no real significance. V/'hen the wave is amplified, limited 4 an inverter stage. It will be apparent that the tubes I0 and II are alternately conductive, so that in the absence of a space signal, a mark signal will be derived as output from the cathode of tube I2. It is common practice inthe electronic computer art to perform gating operations by means :of synchronization pulses. For example, reference is made rto Patent No. 2,540,654, to A. Cohen et al., wherein pulse trains `for synchronizing the operation of playback gates are produced by a pulse generator operating under control of recordings in a timing or synchronizing track of a magnetic drum. A simple exemplification of the manner in which such and shaped this dip disappears and two adjacent half-cycle peaks of the same polarity are bridged.

YCurve c shows -the wave after limiting and -shaping. Curve d shows the same wave slightly delayed by the inherent action of the several stages of ampliiication, the curve itself being arepreferably chosen for the gating yor pulses into any" desired utilization device.

The black ff" spots Which are intersected by theseV lines in curve d'showhow the plus and minus signals Amay bediiferentiated. The lpossibility of this differentiation will be clearly seen as attributable to the-delay laction of the amplifier stages. The timing of such a gating action is coincident with theI passage of keach magnetization spot center under the pole pieces of the playback head.`

Referring now to Fig. 2, I show therein a play b ackhead I positioned to scan the periphery of a magnetic drum 2. The winding of Vthe playback head may be grounded at one end and may feed into the input circuit of an amplifier sta-ge 3. This amplifier stage may, if desired, be followed by other stages 4, 5 and I`6. The numberV 9. This-voltage divider is connected 'be-tween a`4 direct current source of 200 volts, v.f'oreiia'mifla the positive terminal being, of course, -a feeder vfor the anode. The cathode of this stage will be understood to be grounded and a normal 0-volt I grid bias is obtained atlthe mid-'point of this voltage divider. The most negative section 9, of the aforementioned voltage divider constitutes a grid bias'resistcr for a cathode follower tube I IJ. The cathode of tube 10 is connected through a resistor Ii, to a biasing `source terminal of -20 v. and is directly connected to a utilization circuit which is intended to respond to O-volt Ypulses in representation oi space signals.

Mark .signals-may al'sobe obtained by the 'use of another cathode follower tube I I having simi fiarfcircuit parameters to those Vor tube mi, but

l'being'controlled by the latter and so constituting I "gating and pulse -generating means are applicable to the meansof the present invention is shown inFig. 2 wherein the mark and space output lines from tubes I0 and I2 are applied to coincidence gates 20 which are opened at the appropriate times by pulses from a pulse generator 2| synchroni-zed with 4the drum. When magnetic recorders are 'required for recording and playback purposes, these synchronizing pulses may be used again and again for ydiierent switching operations between Ione and another of the component sections of a compu-ter. Itwill bev apparent that there is considerable vadvantage in'being able to perform these switching operations so that one `chain of synchronizing pulses may be utilized lfor all gating purposes. This ladvantage is of the essence of my invention. It will 'also be apparent from the foregoing description of Imy invention that when the gating operations `areperformed coincidently with respect to scanning the spot centers on a magnetic record, there vis no need to provide more 'than lon-ev chain ofv synchronized .gating pulses. In 4other Words, there 'is no `need to :consider that one gating `operationfneed vhe .delayed with respect' to another gating V operation. Y

I claim: Y

1. A playback circuit for magnetic recordings comprising a .scanning .head Ainwhich voltages are `induced upon scanning the magnetization spots of a coded data record, `said scanning headv having its two poles Aseparated'by a'small gap, means for amplifying and. wave-shaping said voltages, 'a source of sharply defined gating pulses,A and means responsive to said gating 'pulses -for differentiating said voltages as to their 'components` of mark and space signicance, saidmeans 'being loperative in substantial coincidence with thepassage of each sensed magnetization spot center past the gap between pole pieces. of said. scanning head.' f

2. In a playback circuit `for deriving 'informa-l tion from a spot-magnetized record track, a scan# kning head and amplification circuit therefor, a source of sharply deiined gating pulses synchronized with the passage of magnetization spot centers past the magnetic aX-isof 'saidscanning'he'ad`, and rreans including Vamplifier structure having an inherent delay .characteristic for delivering output potentials at the yinstants kof said gating pulses, these output potentials being of a high or a `low level depending on vthe polarity ofthe scanned magnetization spot.

3. Means for sensing Aa magnetic recording. track having a series ofmagnetic spots of alternate polarities .densely 'packed therein with the fringes of `a `liacrentspots overlapping., including -a playback head scanning the track, an ampli- ;fying .and pulse shaping circuit driven byusaid head and having a time delay characteristic equal .to the time delaybetween. vpresentaltion ofthe advance fringe land. of the center ofy a spot to the playback head,fand gating means conn trolled to permit output from saidr circuit only in coincidence with the presentation of a spot center to the playback head.

l f HOWARD M. FLEMING,Y Jn.

- REFERENCES CITED Thevfollowing references are of record in the le of 'this patent:

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